In recent years, optical fibers have been used in optical strain gauge systems for sensing the strain, or stress, placed on a subject object. This technology is referred to as optical phase interrogation technology. U.S. Pat. No. 8,346,032 by Schilling, et al., which is assigned to the assignee of the present application, is directed to an optical strain gauge system used to perform optical phase interrogation to determine the strain that has been placed on a subject object. The subject object may be, for example, a concrete piling used in a building, a tower, a rotor blade of a windmill, or a wing of an airplane.
A portion of a strain-sensing fiber, which is typically a plastic optical fiber (POF), is embedded in or attached to the subject object. The embedded portion of the strain-sensing fiber is typically referred to as a meander of fiber. Typically, an adhesive material such as epoxy is used to attach the strain-sensing fiber to the subject object. The ends of the strain-sensing fiber are optically coupled to measurement equipment of the strain gauge system. A reference optical fiber that is identical to the strain-sensing fiber is typically laid alongside the strain-sensing fiber on or in the subject object. The ends of the reference fiber are also optically coupled to the measurement equipment.
A laser diode or a light emitting diode (LED) of the measurement equipment is modulated to produce a modulated light beam. An optical splitter of the measurement equipment splits the modulated light beam into first and second modulated light beams, which are then optically coupled into the first ends of the strain-sensing fiber and the reference fiber. The first and second modulated light beams propagate along the two fibers and pass out of the second ends of the fibers. The measurement equipment includes first and second optical sensors that receive the respective light beams and convert the respective light beams into respective electrical signals. Electrical circuitry of the measurement equipment processes the electrical signals to determine the phase difference between them. The phase difference is then used to determine the difference in the lengths of the two fibers. The extent of the elongation may be used to characterize the strain or stress that has been placed on the subject object, which, in turn, may be used for a number of reasons, such as to determine the integrity of the subject object.
In some cases, the strain-sensing fiber is embedded in a bulk matrix material. The bulk matrix material is either attached to the subject object or is integrated directly into the bulk material of the subject object. The strain that bulk matrix material is subjected to is transferred into the strain-sensing fiber. The phase difference between the signals passing out of the ends of the reference and strain-sensing fibers is determined and used to determine the extent of elongation of the strain-sensing fiber. The extent of the elongation is then used to characterize the strain or stress that has been placed on the subject object.
One of the limitations of this approach is that the bulk matrix material must be carefully chosen for the specific application for which the strain gauge system will be used. For example, physical characteristics of the bulk matrix material such as elastic modulus (E-modules) and coefficient of thermal expansion (CTE) need to be carefully chosen for each specific application to ensure that the bulk matrix material is suitable for use with the subject object. Another limitation of the strain gauge systems described above is that the minimum bend radius of the fibers is so large that the fiber meanders that are attached to the subject object are very large, which increases the size of the strain gauge and makes it unsuitable for use in small areas of a subject object. Also, the length of the fibers that are used in the strain gauge is limited by the minimum bend radius due to the increase in area that is used by a longer fiber meander having a bend radius that is equal to or greater than the minimum bend radius. This limitation in fiber length limits the sensitivity of the strain gauge system. The limitation in sensitivity can limit the accuracy of the strain measurements.
Another limitation of strain gauge systems that embed the strain-sensing fiber in a bulk matrix material is that repeatability is difficult due to the very tight tolerances that must be met in providing a suitable fiber and a suitable bulk matrix material and embedding the fiber in the bulk matrix material. In addition, some sensitivity is lost as the strain is transferred from the bulk matrix material to the fiber jacket, from the fiber jacket to the fiber cladding, and from the fiber cladding to the fiber core. Again, the loss in sensitivity can lead to inaccuracies or insensitivities in the strain measurements.
Accordingly, a need exists for a strain gauge system that is not limited in size by the minimum fiber bend radius and that can be very compact, that is highly sensitive to strain and very accurate, that can be easily made with very tight tolerances and with high repeatability, and that can be made in low to high volume at low costs.